On Wed, Jan 15, 2020 at 10:25 AM Peter Eisentraut <peter.eisentr...@2ndquadrant.com> wrote: > Well, if the transaction was declared read-only, then committing it > (directly or 2PC) shouldn't change anything. This appears to be a > circular argument.
I don't think it's a circular argument. Suppose that someone decrees that, as of 5pm Eastern time, no more read-write transactions are permitted. And because the person issuing the decree has a lot of power, everybody obeys. Now, every pg_dump taken after that time will be semantically equivalent to every other pg_dump taken after that time, with one tiny exception. That exception is that someone could still do COMMIT PREPARED of a read-write transaction that was prepared before 5pm. If the goal of the powerful person who issued the decree was to make sure that the database couldn't change - e.g. so they could COPY each table individually without keeping a snapshot open and still get a consistent backup - they might fail to achieve it if, as of the moment of the freeze, there are some prepared write transactions. I'm not saying we have to change the behavior or anything. I'm just saying that there seems to be one, and only one, way to make the apparent contents of the database change in a read-only transaction right now. And that's a COMMIT PREPARED of a read-write transaction. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
